One-channel EL84 reverb amp with tremolo, late 1960s

Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.

How to use the bench

Every part is simulated. Nothing you hear is a recording.

  1. 1Pick a circuit

    Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.

  2. 2Set the knobs and the input

    Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.

  3. 3Press Play

    Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.

  4. 4Tap a part

    Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.

  5. 5Tap a wire

    Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.

  6. 6Try a change

    Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.

Good to know

Modeled after the Gibson Hawk.

One 12AX7 stage into Tone and Volume, a notch filter and the reverb return into the driver, then a cathodyne into two cathode-biased EL84s. The tremolo swings both output grids through their grid leaks.

Input

Add

Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.

How it’s played
Tap a part to change its value, or a wire to probe it. To add or take out parts, open Modify this circuit, below. Red numbers are DC voltages to ground, not warnings. Scroll or drag sideways to see the whole circuit.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.

Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.

Add a part

Tap where each leg goes. Tap both ends of a part to put the new one across that part.

Change a part

With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.

Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.

Change a wire

With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.

With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.

Keep and share

Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.

Test tone for the graphs
The graphs use a steady tone. Play sounds a plucked string that dies away.

Input

pickup, unloadedat the pedal input

Output

at the amp input

Probe

Spectrum

Frequency response

What the curves mean

The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.

Output

Bias

every voltage and current at rest

What to listen for. One 12AX7 stage with a light cathode bypass, into a .001 µF coupling cap, so the deep bass is trimmed right at the start. Tone is a .0047 µF to ground through a variable resistor. After Volume a 220 kΩ / 220 kΩ filter with .0047 µF to ground and 500 pF across it makes a notch near 470 Hz. The reverb return joins at the driver’s grid through 470 kΩ, then a cathodyne splits the signal for two EL84s on a shared 100 Ω, with 4.7 kΩ of feedback from the speaker. The tremolo oscillator’s swing goes through .047 µF and 1 MΩ to where the output grids’ leaks meet, so both grids move together. There’s no depth control. The springs are a measured tank, played from its impulse response. The tube models are typical of the types, the transformers typical figures, not measured.

Download.

Parts and what they do

Stock values. Tap a part on the schematic to change it.

R1A, 47 kΩ
Input 1 resistor, 47 kΩ into the grid. Input 2 has no switch, so a guitar in input 1 sees only this one.
R1B, 47 kΩ
Input 2 resistor, 47 kΩ into the grid. With nothing in input 1 that jack grounds its own tip, so the two 47 kΩ make a divider and input 2 is about 6 dB quieter.
V1b, 12AX7 / ECC83 / 7025
One half of the first 12AX7, the only gain stage ahead of the driver. The other half is the driver in the power amp.
270k, 270 kΩ
Plate resistor for V1b. The drawing reads +150 V on the plate.
4.7k, 4.7 kΩ
Cathode resistor for V1b, +1.5 V on the drawing. It runs the stage cool, about a third of a milliamp.
.2µ, 200 nF
Only .2 µF across the cathode resistor, so the stage has full gain only from about 170 Hz up. Below that the cathode resistor lowers the gain.
.001µ, 1 nF
Only .001 µF to the tone node. Into the Volume pot and the reverb driver’s grid it trims the low bass.
.0047µ, 4.7 nF
The tone cap. With the Tone control under it, it bleeds treble from the tone node to ground.
Tone, 1 MΩ pot, audio taper
Tone, a 1 MΩ pot as a variable resistor under the .0047 µF. Turned up, its resistance rises, less treble goes to ground and the amp gets brighter. Taper typical, not on the drawing.
Vol 1M, 1 MΩ pot, audio taper
Volume, 1 MΩ. Its wiper feeds the notch filter and the driver. Taper typical, not on the drawing.
220k, 220 kΩ
First half of the filter from the Volume to the driver’s grid.
220k, 220 kΩ
Second half of the filter, into the driver’s grid.
.0047µ, 4.7 nF
From the middle of the two 220 kΩ to ground, 5 % on the drawing. With the 500 pF across both it makes a bridged-T notch near 470 Hz, a deep dip in the low mids.
500p, 500 pF
Across both 220 kΩ. It carries the treble past the filter.
470k, 470 kΩ
Grid resistor for the reverb driver, from the tone node to ground.
V2a, 12AX7 / ECC83 / 7025
One half of the second 12AX7, the reverb driver. Its grid is the tone node itself. The drawing reads +1.4 V on its cathode.
1.5k, 1.5 kΩ
Cathode resistor for the reverb driver.
.1µ, 100 nF
Across the reverb driver’s cathode resistor, so the driver has full gain only in the treble.
T2, 25 kΩ to 8 Ω, 20 H primary, 100 mH leakage
The reverb transformer, from the driver’s plate to the spring unit. It loads the driver and the supply. Typical of a small 12AX7-driven reverb transformer, not on the drawing.
Unit in R, 800 mΩ
Resistance of the reverb unit’s input coil. The current through it drives the springs, which play from a measured tank’s impulse response. Typical of a long spring tank, not measured.
Unit in L, 1.3 mH
Inductance of the reverb unit’s input coil. Typical of a long spring tank, not measured.
Unit out L, 360 mH
The reverb unit’s output coil, from the recovery grid to ground. The springs come back in series with it. Typical of a long spring tank, not measured.
Unit out R, 200 Ω
Resistance of the unit’s output coil. Typical, not measured.
220k, 220 kΩ
Grid resistor for the recovery stage.
V2b, 12AX7 / ECC83 / 7025
The other half of the second 12AX7, the reverb recovery stage. It brings the springs back up.
220k, 220 kΩ
Plate resistor for the recovery stage. The drawing reads +65 V on the plate.
470, 470 Ω
Cathode resistor for the recovery stage, +.49 V on the drawing.
10µ, 10 µF electrolytic
Bypass for the recovery stage’s cathode.
.0047µ, 4.7 nF
Coupling cap to the Reverb control, small, so only the springs’ upper range comes back.
Reverb, 500 kΩ pot, audio taper
Reverb, 500 kΩ. Its wiper feeds the driver’s grid through 470 kΩ, and it’s that grid’s path to ground. Turn it up for more of the springs. Taper typical, not on the drawing.
470k, 470 kΩ
From the Reverb control to the driver’s grid. It’s also the grid’s DC path to ground.
150k, 150 kΩ
Plate resistor for the oscillator. The drawing reads +165 V on the plate.
V3a, 12AX7 / ECC83 / 7025
One half of the third 12AX7, the tremolo oscillator, its cathode grounded. The other half is the cathodyne in the power amp.
.033µ, 33 nF
First cap of the oscillator’s phase-shift ladder.
68k, 68 kΩ
In series with the speed control, the ladder’s first leg. It sets the top speed.
Speed, 1 MΩ pot, reverse audio taper
The Tremolo Control, 1 MΩ as a variable resistor in the ladder’s first leg: the speed. Turned up, its resistance drops and the oscillator runs faster. Its far end goes through the foot switch to ground. Reverse audio taper, typical (the drawing marks none).
.02µ, 20 nF
Second ladder cap.
510k, 510 kΩ
Second ladder leg to ground.
.02µ, 20 nF
Third ladder cap, into the grid. The three caps and legs turn the plate’s swing round by half a cycle at one frequency, so it oscillates there.
1.5M, 1.5 MΩ
Grid leak for the oscillator, and the ladder’s last leg.
.047µ, 47 nF
Couples the oscillator’s swing from its plate and blocks the plate’s DC.
1M, 1 MΩ
From the cap to where the output grids’ leaks meet. With the 220 kΩ below it, it sets how much of the swing reaches the grids. There’s no depth control.
220k, 220 kΩ
From the output grids’ return to ground, the grids’ DC path. With the 1 MΩ above it, it passes about a sixth of the oscillator’s swing to both grids.
V1a, 12AX7 / ECC83 / 7025
12AX7, the driver. A cathodyne has no gain of its own, so this stage raises the preamp’s signal about 30 times, enough to drive the output tubes. Its grid connects straight to the preamp’s output with no coupling cap, so its path to ground is through the preamp’s volume and tone network.
100k, 100 kΩ
Plate resistor for the driver.
1k, 1 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or so. It sits on the feedback leg, so the fed-back signal moves the driver’s cathode.
5µ, 5 µF electrolytic
Bypass cap across the driver’s cathode resistor, for full gain above a few hertz. It goes to the top of the feedback leg, not to ground, so the feedback still reaches the cathode.
.01µ, 10 nF
Coupling cap from the driver to the cathodyne.
V3b, 12AX7 / ECC83 / 7025
The cathodyne, or split-load inverter. The same current runs through its plate resistor and its cathode resistors, which are about equal, so the plate swings one way and the cathode the other by about the same amount. The cathode drives tube A, the plate tube B. Its gain is slightly under 1 on each side.
150k, 150 kΩ
The cathodyne’s plate resistor, the output to tube B.
3.3k, 3.3 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
150k, 150 kΩ
The cathodyne’s cathode load, the same value as its plate resistor. The cathode above the bias resistor is the output to tube A.
470k, 470 kΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage.
4.7k, 4.7 kΩ
The feedback resistor from the speaker. With the 100 Ω below it, it sends about 2 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
100, 100 Ω
The bottom of the feedback loop, 100 Ω to ground under the driver’s cathode resistor. It isn’t bypassed, so the fed-back speaker voltage moves the cathode. The driver’s current runs through it too, so it’s kept small.
1µ, 1 µF electrolytic
A 1µF cap across the bottom of the feedback loop, as on the drawing. Above about 1592 Hz it shorts the leg, so less of the treble is fed back and the top end comes up.
.01µ, 10 nF
Coupling cap to tube A’s grid.
.01µ, 10 nF
Coupling cap to tube B’s grid.
1k, 1 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1k, 1 kΩ
Grid stopper for tube B.
V4, EL84 / 6BQ5 (Philips curves)
EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, EL84 / 6BQ5 (Philips curves)
EL84, tube B of the pair.
220k, 220 kΩ
Grid leak for tube A, returned with tube B’s to the tremolo’s Depth pot, so the oscillator swings both grids together. The bias comes from the cathode resistor. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
220k, 220 kΩ
Grid leak for tube B.
100, 100 Ω
The shared cathode resistor. Every output tube’s current runs through it and lifts the cathodes above the grids, and that’s the bias. Driven hard, the pair draws more current on average, so the cathodes climb and the tubes run colder, which softens the clipping.
100µ, 100 µF electrolytic
Bypass cap across the cathode resistor, so the cathodes hold still at the signal’s frequencies. It charges and discharges slowly as the average current changes on a hard note.
1k, 1 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the inverter and preamp, as on the Gibson Hawk drawing. The current drawn past it drops the voltage, so each stage down the chain runs lower than the one before, and a hard note that pulls B+ down pulls these down too. The preamp runs off this node instead of its own HT supply.
20µ, 20 µF electrolytic
Filter cap at the node for the inverter and preamp. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 8 kΩ to 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 8 kΩ plate to plate into 8 Ω, center tap on B+. Tap any piece of it to change its figures. The core saturates as the flux nears its knee. Every figure is typical of the part, not measured.
PT, 243 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 243 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part, not measured.
PT, 243 V ac, 60 Hz
The other half of the winding, from the grounded center tap. It runs opposite the first half, so the rectifier charges the reservoir on every half cycle.
PT R, 60 Ω
Resistance of one half of the winding, plus the primary’s share. With the rectifier’s own drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 60 Ω
Resistance of the other half. Typical of the part, not measured.
D1, 1N4001
One of the silicon rectifier diodes. It drops under a volt, so B+ holds up better under load than with a tube rectifier.
D2, 1N4001
The other rectifier diode, on the other half cycle.
40µ, 40 µF electrolytic
The reservoir cap. It feeds the output transformer’s center tap. It charges in short pulses at each peak of the mains and drains between them, so B+ carries ripple, and on a hard note it sags.
Coil R, 8.69 Ω
Voice coil resistance, the floor of the speaker’s impedance. The shape of a 1x12 open-back cab with an alnico 12 in, measured (a 93 Ω peak near 74 Hz on 8 Ω), scaled to the 8 Ω tap. One real cab, not every speaker. The Speaker menu by the play buttons is separate. It plays the note through a speaker and cab impulse response, with either load.
Coil L, 256 µH
Voice coil inductance. It makes the impedance rise through the treble. The feedback holds the output steadier against that than it would be without feedback.
Loss L, 464 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 14.3 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 2.13 mH
A second step in the treble rise, with Loss R2 across it: a real voice coil’s losses keep climbing from a few hundred hertz.
Loss R2, 2.19 Ω
Limits what Loss L2 adds.
Tank R, 83.7 Ω
Height of the bass resonance, about 92 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 21.1 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 220 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

As drawn
All three 12AX7s use the 12AX7 model. The reverb transformer and unit are typical, not on the drawing.

How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type, not measured parts. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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